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Author Topic: 50 Buck bot Battery Question  (Read 1078 times)

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Offline lacanauTopic starter

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50 Buck bot Battery Question
« on: September 10, 2010, 11:58:51 PM »
Hi everyone I'm building the 50 dollar robot.
Bought my 6V Ni-mH battery the other day, at 1600mA

The charger I bought seperately is 6V at 500mA
I was testing the circuit for the first time, (it didnt smoke! yey!) and I noticed I was reading about 3V all around, I then checked the voltage directly from the battery and it said 3V too.

So I plugged in the charger and connected the battery. Immediately the light flashes indicating trickle charging (this means the battery is full, i think....)
What am I missing here?

A side note: The guy at the store said that the charger is originally meant for charging Gel batteries, but says it works with both.

Some pictures in case you couldn't quite follow:





Cheers all

Offline Soeren

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Re: 50 Buck bot Battery Question
« Reply #1 on: September 11, 2010, 12:51:07 AM »
Hi,

A 5-cell NiMH battery needs to be charged to 7.0V to 7.25V for a full charge and the charger has to put out a slight bit more to negotiate contact resistance.

That's not your (entire) problem though, as 3.107V indicates either two overcharged cells, 3 flat cells or 4 to 5 seriously discharged cells.
If one cell has been pole reversed (by being discharged way too low and/or being very unbablanced to start with) it will take down the measure by two cells.

Another possible reason is a too high cell impedance (eg. due to old age and/or overcharge). If you have one cell pole reversed and a bit of high impedance (which will make a flat battery seem fully charged very quickly, without it being the case), the 3.107V is quite possible.

But... This is all guessing (though with decades of practice :)), so first you have to check each cell one by one, by piercing through the isolation material - sewing needles may help, but don't short any cells!
When you have the exact voltage of each cell and its polarity (neighboring cells will be placed with the positive poles facing opposite), come back with the numbers for a solution.

While you're at it, measure the chargers output, unloaded and while charging.


I do fear the battery is beyond a full life as a 5-cell battery, but if that's the case, the good ones may still be used for other purposes.


Never trust a sales persons word  ;D
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

Offline knossos

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Re: 50 Buck bot Battery Question
« Reply #2 on: September 11, 2010, 01:06:36 AM »
Never trust a sales persons word  ;D

Hehe, ain't that the truth.  lacanau, if you want to read more about batteries, a fairly informative page can be found here:

http://www.camlight.com/techinfo/techtips.html

While there are quite a few posts on SoR about batteries and chargers, this post talks a bit about battery chargers for NiMH batteries and might also be of use to you:

http://www.societyofrobots.com/robotforum/index.php?topic=319.0

[TLDR]  For NiMH batteries:
1. Make sure your charger is rated at a higher voltage than your battery
2. To prolong battery life you should charge at rate of about 1/10 the mAh rating: about 160mA for  a 1600 mAh battery.
3. New batteries or batteries that have been dormant need several conditioning charges
4. Don't overcharge your batteries, it can kill them.

« Last Edit: September 11, 2010, 01:15:26 AM by knossos »
"Never regret thy fall,
O Icarus of the fearless flight
For the greatest tragedy of them all
Is never to feel the burning light."
 
— Oscar Wilde

Offline lacanauTopic starter

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Re: 50 Buck bot Battery Question
« Reply #3 on: September 11, 2010, 04:29:00 AM »
Hi guys cheers for the sweet info rly appreciate it.
Couple of other things...

After I wrote my first post I hooked up the battery for about 10 minutes after which I read a voltage of ~6.7
I then disconnected it and checked again a few hours later, voltage of about 6.1.

I also forgot to mention I bought this battery 2 days ago  :P

How much higher should the voltage output of the charger be for the 6V batt to be 'optimal'

I've also got a lot of transformer copper around, is it that difficult to go custom?

thanks again

Offline knossos

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Re: 50 Buck bot Battery Question
« Reply #4 on: September 11, 2010, 09:01:56 AM »
Considering volts per cell shouldn't exceed about 1.8v and you have a 5-cell battery pack, I would think anything above about 9v (1.8 * 5) would be unnecessary.

Also, I would say going custom would be more trouble than it's worth.

If you are interested in going custom, you might want to read more about NiMH batteries and chargers.  A few links to help get you started:

http://www.panasonic.com/industrial/includes/pdf/Panasonic_NiMH_Overview.pdf
http://www.panasonic.com/industrial/includes/pdf/Panasonic_NIMH_Precautions.pdf
http://www.panasonic.com/industrial/includes/pdf/Panasonic_NiMH_ChargeMethods.pdf
http://www.panasonic.com/industrial/includes/pdf/Panasonic_NiMH_ChargeMethods.pdf
http://www.panasonic.com/industrial/includes/pdf/Panasonic_NiMH_BatteryPacks.pdf
http://www.camlight.com/techinfo/whydischarge.html
http://www.powerstream.com/NiMH.htm
http://www.cobasys.com/pdf/tutorial/inside_nimh_battery_technology.pdf

And as always, Google is your friend :)
"Never regret thy fall,
O Icarus of the fearless flight
For the greatest tragedy of them all
Is never to feel the burning light."
 
— Oscar Wilde

 


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